This invention relates to the coating of steel substrates with metallic coatings and more particularly to the coating of steel products having substantial cross sectional areas with aluminum-zinc coatings.
Aluminum-zinc coatings and the products of such coatings have been disclosed and claimed in U.S. Pat. Nos. 3,343,930 and 3,393,089 issued to the present applicants in 1967 and 1968 repsectively. While the coatings disclosed in these prior patents were known to have very desirable corrosion properties it has since been determined by long term corrosion tests that the corrosion properties of the coatings are even more desirable than had at first been suspected. It has also been discovered that the optimum commercial corrosion properties of such coatings occur in a range centering rather closely about 55% aluminum and 45% zinc, at least for linear type relatively thin ferrous materials such as sheet, strip and wire products. Such products, which can be defined as bendable or inherently formable linear products require, when made by a hot dipping process, as taught previously by the prior patents, at least 0.5% silicon in the molten coating bath in order to attain a ductile adherent coating on the linear product. Bendable or inherently formable sections are customarily less than a quarter of an inch in thickness.
It has been discovered, however, that when aluminum-zinc type coatings are applied to more massive low alloy ferrous type products such as structural sections, plate, bars, castings, forgings and machined shapes and the like, where the dipping time is normally somewhat extended and the freezing time usually extended by the reduced rate of cooling of the more massive cross section of the product, that the intermetallic alloy layer between the outer coating and the underlying steel substrate tends to excessive growth. While the coatings desired on structural sections and the like are frequently thicker than the coatings desired for sheet and strip and the like, it is undesirable for such heavier coatings to be composed entirely of an alloy containing significant percentages of iron if the coating is to be smooth, adherent and have a bright metallic lustre. If the intermetallic layer thickens or grows to the extent that portions of the intermetallic layer extend to the surface of the overlying aluminum-zinc coating the smoothness and bright metallic lustre of the coating will be destroyed because of the uneven dull nature of the intermetallic layer. It has, therefore, been discovered that when coating massive steel sections with aluminum-zinc coatings that in order to obtain a smooth coating with a bright metallic lustre the amount of silicon present in the molten coating bath must be greater than the minimum necessary for the coating of linear products such as sheet, strip and wire.
It has also been discovered that for some purposes it may be advantageous to use even larger amounts of aluminum in the coating than the maximum set for the most desirable corrosion, i.e., not more than 70% aluminum as taught in applicants' prior patents. For more massive structural sections and the like it may be quite satisfactory to use more aluminum in the coating and at the same time have a thicker coating, as is usual in any event in structural sections, in order to make up for the loss in corrosion resistance. This is particularly true as the price of zinc becomes relatively greater than the price of aluminum on the world market and the use of relatively larger percentages of aluminum in the coating becomes more economically desirable. It has also been determined, however, that the very vigorous exothermic reaction which is characteristic of the application of aluminum-zinc coatings to steel and other high ferrous substrates reaches a maximum at about 70% zinc and then declines rapidly and dies out at about 85% aluminum. It has been discovered that this exothermic reaction, the vigorousness of which can be measured roughly by the elevation of the temperature of a sample above the temperature of a coating bath in which a sample has been immersed after the sample is removed from the bath, begins at a percentage of aluminum in the molten bath of about 25%, rapidly rises to a maximum at about 70 to 80% aluminum content in the bath where the rise in temperature of the sample after removal from the bath is about 300.degree. Fahrenheit and then drops precipitously back to a minimum at about 85% aluminum content.
This vigorous reaction of the substrate with the molten aluminum-zinc coating results, unless the proper precautions are taken, in excessive growth of alloy layer on such heavy sections. In addition if high aluminum content aluminum-zinc coating baths are used, for example, in the range of 70 to 85% or so aluminum, the vigorous reaction of the substrate with the bath may result in the production of an excessive growth of alloy layer with resulting poor adhesion and ductility as well as a lack of a smooth and bright coating.